1. Field of the Invention
The present invention relates to a mobile communication system, and in particular, to an apparatus and a method for transmitting/receiving a control channel message in a mobile communication system.
2. Description of the Related Art
In general, wireless communication systems have been developed to handle situations in which it is impossible to connect a fixed wire network up to a remote terminal. Examples of such wireless communication systems include various mobile communication systems, such as a wireless Local Area Network (LAN) system, a Wireless Broadband Internet (WiBro) system, a mobile Ad hoc system, an Ultra Mobile Broadband (UMB) system, etc.
A mobile communication system is based on the mobility of a user as opposed to a simple wireless communication system, where the ultimate object of a mobile communication system is to enable exchange of information media between people any time and anywhere by using a Mobile Station (MS), such as a mobile phone, a Personal Data Assistant (PDA), or a wireless pager. With rapid development of communication technology, the current mobile communication systems can provide, through a mobile station, not only conventional voice communication services, but also high speed data services capable of transmitting large capacity digital data, such as moving images, for example, as well as still images or even E-mail.
A representative example of a mobile communication system that provides high speed data services using multi-carrier transmission schemes is an Orthogonal Frequency Division Multiplexing (OFDM) system. OFDM is a scheme for converting a serially input symbol stream into parallel signals and then transmitting the converted signals through multiple sub-carriers having a mutual orthogonality. The OFDM method of data transmission came into the spotlight around the time of the development of Very Large Scale Integration (VLSI) technology in the early 1990's.
The OFDM scheme modulates data by using multiple sub-carriers between which the mutual orthogonality is maintained. The OFDM scheme is more robust against a frequency selective multi-path fading channel than a conventional single carrier modulation scheme. Therefore, the OFDM scheme is proper for a high speed packet data service.
In order to overcome the fading phenomenon in the OFDM system, the following two types of technology are used at the time of channel transmission, Adaptive Modulation and Coding (AMC) technology and diversity technology.
First, according to the AMC technology, a modulation scheme and a coding scheme are adaptively adjusted according to the channel change of a downlink. In a typical OFDM system, a plurality of AMC sub-bands are defined, and each sub-band independently performs the AMC operation. Usually, multiple sub-carriers necessary for transmission of data to a predetermined mobile station are adjacent to each other. Therefore, the AMC technology can transmit data through an AMC sub-band including multiple adjacent sub-carriers having a good channel response characteristic and is thus proper for a traffic transmitted to a predetermined user.
Second, according to the diversity technology, a traffic is transmitted in a frequency domain and through resources distributed in a time domain. The characteristic of a wireless channel changes in various ways along the time axis, and fluctuation of the channel characteristic between good and bad channel characteristics is repeated along the frequency axis. The diversity technology is targeting to enable the link with a mobile station in a wireless environment as described above to experience both a good channel environment and a bad channel environment as equally as possible. The diversity technology is proper for transmission of a traffic sensitive to the delay or a traffic of a channel commonly used by a plurality of users.
As described above, the two types of technology used in order to overcome the fading phenomenon in the OFDM system, that is, the AMC technology and the diversity technology, have characteristics showing a contrast to each other and have different types of traffics proper for use. Therefore, it is necessary to operate a system while properly mixing the two types of technology instead of applying only one type of technology to an OFDM system. In the OFDM system, a resource transmitted according to the AMC technology is called a Distributed Resource Channel (DRCH).
In the OFDM system, according to the allocation scheme of frequency resources (i.e. sub-carriers), the channel type of a traffic channel can be classified into DRCH for allocating sub-carriers in a distributed manner and Block Resource Channel (BRCH) for adjacently allocating sub-carriers. According to the DRCH, tones allocated in order to obtain the frequency diversity as much as possible are distributed over the entire frequency band as widely as possible. In contrast, in order to facilitate interference estimation, the BRCH includes adjacent tones in the frequency axis. Methods for multiplexing the above two types of channels can be divided into two multiplexing (hereinafter, mux) modes of a resource channel like the examples illustrated in FIGS. 1 and 2.
FIGS. 1 and 2 illustrate examples of resource allocation by mux modes of DRCH and BRCH in a typical OFDM system. Each of FIGS. 1 and 2 illustrates one physical channel (PHY) including eight OFDM symbols.
FIG. 1 illustrates a first mode for multiplexing and transmitting a DRCH and a BRCH (hereinafter, mux mode 1). In the mux mode 1, resources 110 to be allocated to the DRCH are appointed, and resources 120 to be allocated to the BRCH are then appointed. At this time, the resources 110 to be allocated to the DRCH cannot be reallocated to the BRCH. In the mux mode 1, a common pilot is used as a pilot channel for channel estimation and data demodulation, for example, as a Forward Link-Common Pilot Channel (F-CPICH).
FIG. 2 illustrates a second mode for multiplexing and transmitting a DRCH and a BRCH (hereinafter, mux mode 2). In the mux mode 2, the entire frequency resources are divided into a plurality of sub-bands, which include a DRCH sub-band 220 usable as the DRCH and a BRCH sub-band 210 usable as the BRCH. For channel estimation in the mux mode 2, the DRCH sub-band 220 uses a common pilot as in the mux mode 1, and the BRCH sub-band 210 uses a dedicated pilot.
FIG. 3 illustrates an example of a dedicated pilot format of the BRCH in the mux mode 2.
In FIG. 3, pilot formats 310, 320, and 330 have a tile structure including 16 tones along a frequency axis and 8 OFDM symbols along a time axis. Reference numeral 302 denotes a pilot tone, and reference numeral 304 denotes a data tone used as a control channel or a data channel. In the mux mode 2, dedicated pilot tones used in the case of the BRCH use the same power as that of neighbor data tones transmitted to each user, differently from the common pilot used in the mux mode 1. Which one will be used from among the formats of the dedicated pilot is determined based on either a speed of a mobile station or if Multiple-Input Multiple-Out (MIMO) is used.
In a typical OFDM system, format information of a dedicated pilot is transmitted to a mobile station through a Forward Link Shared Control Channel (F-SCCH), which is a control channel of a forward link. The F-SCCH carries various control channel messages, and a format of a dedicated pilot is carried from a base station to a mobile station by one field of a Forward Link Assignment Message (FLAM), which is one of control channel messages. However, format information of a dedicated pilot is necessary only in the case of transmitting the BRCH in the mux mode 2 and is unnecessary in other cases. Therefore, if one field of the FLAM is always used as format information of a dedicated pilot, it may be a wasteful use of resources of the control channel.